Primary side PFC driver with dimming capability

ABSTRACT

A primary side PFC driver circuit is disclosed that includes a switch control circuit for commanding a switch to allow an inductor coupled to an output load (e.g., LEDs) to transfer energy provided by an input voltage source. The switch control circuit provides two signals for commanding the switch. A first signal having a first frequency, with a duty cycle in proportion to the input voltage amplitude, commands the switch to allow the average input current to be proportional to the input voltage amplitude. A second signal having a second frequency higher than the first frequency pulses the output load with substantially constant current pulses based on a value of the first signal (e.g., while the first signal is high). The current pulses produce a substantially constant current in the output load.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S.application Ser. No. 13/249,158, entitled “Primary Side PFC Driver withDimming Capability,” filed on Sep. 29, 2011, the entire contents ofwhich are incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates generally to electronics and more particularlyto Power-Factor-Correction (PFC) driver circuits for light emittingdevices, such as a Light Emitting Diode (LED).

BACKGROUND

A switching-mode power supply (SMPS) can be used to drive a string ofLEDs. The SMPS often includes a full-wave rectifier circuit forrectifying an AC input voltage into a DC voltage. The DC voltageprovides input current to the LEDs. The SMPS can include PFC circuitrythat controls the input current so that the input current waveform is inphase with the waveform of the AC input voltage (e.g., a sine wave). Fora good power factor, the input current waveform will have the same shapeand phase as the AC input voltage waveform, but will vary in magnitudeor Root Mean Square (RMS) value. A good power factor can help efficientdelivery of electrical power from the AC input voltage source to theLEDs.

Conventional SMPS circuits for LED drivers include one or two stage PFCconverters, or a primary side driver system. Two stage PFC convertersadd cost and have lower efficiency due to the second stage ofconversion. Single stage PFC converters require large electrolyticcapacitors, which is bulky and unreliable and can shorten the life ofthe SMPS system. Single stage PFC converters are also not suitable forLED dimming applications. Primary side driver systems are susceptible tocolor shifting caused by changing current in the LEDs. In addition,since the system is off for a substantial portion of time the powerfactor is deteriorated.

SUMMARY

A primary side PFC driver circuit is disclosed that includes a switchcontrol circuit for commanding a switch to allow an inductor coupled toan output load (e.g., LEDs) to transfer energy provided by an inputvoltage source. The switch control circuit provides two signals forcommanding the switch. A first signal having a first frequency, withduty cycle in proportion to the input voltage amplitude, commands theswitch to allow the average input current to be proportional to theinput voltage amplitude. A second signal having a second frequencyhigher than the first frequency pulses the output load withsubstantially constant current pulses based on a value of the firstsignal (e.g., while the first signal is high).

Using two signals to command the switch at different frequenciesprovides the one or more light emitting devices with a substantiallyconstant pulse current while controlling the input current so that theaverage input current waveform is in phase with the waveform of therectified input voltage. The constant pulse current prevents the lightemitting devices from color shifting and controlling the shape and phaseof the average input current waveform provides good PFC.

Particular implementations of a primary side PFC LED driver with dimmingcan provide one or more of the following advantages: 1) good PFC in asingle stage; 2) no large electrolytic capacitor (longer life for LEDs);3) no transformer; 4) constant LED current (no color shift); 5) low costimplementation; and 6) dimming capability.

The details of one or more disclosed implementations are set forth inthe accompanying drawings and the description below. Other features,aspects, and advantages will become apparent from the description, thedrawings and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of an exemplary floating buck-boostpower converter circuit for driving light emitting devices.

FIG. 2 illustrates exemplary waveforms for the converter circuit of FIG.1.

FIG. 3 is a simplified block diagram of an exemplary floating buck powerconverter circuit for driving light emitting devices.

FIG. 4 illustrates exemplary waveforms for the converter circuit of FIG.3.

FIG. 5 is a flow diagram of an exemplary process for driving LEDs.

DETAILED DESCRIPTION Exemplary Primary Side Driver Circuit

FIG. 1 is a simplified block diagram of an exemplary floating buck-boostpower converter circuit 100 for driving light emitting devices, such asLEDs 108. In some implementations, converter circuit 100 can includefull-wave rectifier circuit 102 (e.g., a diode bridge), inductor 104 anddiode 106. Converter circuit 100 has an output voltage magnitude that iseither greater than or less than the input voltage magnitude. The outputvoltage is adjustable based on the duty cycle of switch 118 (e.g., aMOSFET power transistor).

While switch 118 is in an “on” state, the rectified AC input voltagesource (e.g., 110 VAC) is connected to inductor 104 and disconnectedfrom LEDs 108 due to switch 118 providing a path to ground. This resultsin accumulating energy in inductor 104. While switch 118 is in an “off”state, inductor 104 is connected to LEDs 108, so current is transferredfrom inductor 104 to LEDs 108. This current is substantially constant socolor shifting of LEDs 108 is avoided. Diode 106 (e.g., a Schotky diode)prevents negative voltage across LEDs 108 during the “on” state ofswitch 118.

In some implementations, switch control circuit 101 can include waveformgenerator 110, logic 112, comparator 114, latch 116 and sense resistor120. Waveform generator 110 can be a full-wave rectified sine wavegenerator that is configured to generate an n-level Pulse WidthModulation (PWM) signal (e.g., 16-64 levels). In some implementations,the phase of the AC input voltage could be used to generate digitally aPWM signal. In other implementations, the AC input voltage can bemeasured with an analog-to-digital converter (ADC) and used as the PWMsignal. In this example, the PWM signal is a 5-level-PWM-of-current(“5LPC”) signal. The full-wave rectified AC input is 120 Hz (2×60 Hz),resulting in the 5LPC signal changing at about 600 Hz to 3000 Hz.

Logic 112 can be an “AND” gate or other suitable combination of logicdevices. Logic 112 has a first input coupled to the output of waveformgenerator 110 and a second input coupled to a second waveform generator(e.g., an oscillator) that generates a second signal for commandingswitch 118. As discussed in reference to FIG. 2, the second signal isprovided to a command terminal of switch 118 during the “on” time of the5LPC signal, and changes in the range of a few hundred kilohertz to afew megahertz. When the 5LPC signal is high, switch control circuit 101is providing switch commands to switch 118. When the 5LPC signal is low,switch control circuit 101 is off.

The output of logic 112 is coupled to a first input of latch 116 (e.g.,a set input of an SR flip-flop). An output of latch 116 is coupled tothe command terminal of switch 118 (e.g., a gate terminal of a MOSFETpower transistor). In this example, the output of latch 116 will remainlatched (e.g., to logic “1”) when the output of logic 112 is high. Thisresults in switch 118 being turned on and inductor 104 being groundedthrough sense resistor 120. The voltage “sensed” across sense resistor120 is input (negative input terminal) to comparator 114.

In implementations that include peak current control, comparator 114compares the voltage across sense resistor 120 to a reference voltageIref coupled to the other input terminal of comparator 114 (positiveinput terminal). Iref can be a constant voltage that sets theswitching-mode supply peak current for LEDs 108. In otherimplementations, average current control circuitry can be used. Theoutput of comparator 114 is used to reset latch 116 when the peakcurrent is reached (e.g., applied to reset input of SR flip-flop). Whenthe peak current is reached, the command voltage applied to switch 118closes switch 118, thereby allowing current to flow from inductor 104 toLEDs 108.

In converter circuit 100, there is no large electrolytic capacitor atthe output, so when switch 118 is “on” to charge inductor 104, LEDs 108are off. Switch control circuit 101 implements a “PWM signal inside aPWM signal” to ensure a substantially constant pulse current flow intoLEDs 108 during all stages of operation of circuit 100, thus preventingcolor shifting in LEDs 108 due to a varying current level when LEDs 108are turned on.

Converter circuit 100 provides various advantages over conventional PFCdriver circuits, including eliminating the large electrolytic capacitor,and providing good PFC and dimming capability in a single stage.Although switch control circuit 101 is shown as implemented withdiscrete components, the functionality of switch control circuit 101could also be implemented in a programmable microcontroller.

Exemplary Waveforms

FIG. 2 illustrates exemplary waveforms for converter circuit 100 ofFIG. 1. In these exemplary waveforms, a 5LPC waveform is assumed. Thewaveforms illustrate a 5LPC approximation that has been controlled tohave same shape and phase as the AC input voltage. The PWM generated bywaveform generator 110 (a sine wave) has a shorter duty cycle when thecurrent is low and a longer duty cycle when the current is high. Forexample, at the peak current, four out four PWM pulses are high.

The waveforms illustrated in FIG. 2 are for a buck-boost topology ofconverter circuit 100. The waveforms illustrate the conversion from ananalog current to a substantially constant current, and the “PWM signalinside a PWM signal,” which provides a substantially constant pulsecurrent to LEDs 108. LEDs 108 are pulsed with substantially constantcurrent pulses, but the average LED current is synced (has the samephase and shape) with the rectified AC input voltage. Since there is nolarge electrolytic capacitor in parallel with LEDs 108, LEDS 108 will beoff when switch 118 is on. The buck-boost topology will work even whenthe AC input voltage is lower than the output voltage and provide goodPFC. Since LEDs 108 are off when inductor 104 is storing energy, theaverage current (brightness) of LEDs 108 will be smaller. The peakcurrent flowing in LEDs 108 can be increased (e.g., increase Iref) tocompensate for this effect.

LEDs 108 in floating buck-boost topology are only directly powered wheninductor 104 is discharging its energy. This means that LEDs 108 will beoff when inductor 104 is storing energy and the average inductor currentwill be smaller. This effect can be compensated by increasing the peakcurrent of inductor 104. If it is desired to reduce the peak LED currentto the average current, a small capacitor can be placed in parallel withLEDs 108 to provide continuous current to LEDS 108. For example, a 5 uFcapacitor placed in parallel with LEDs 108 will allow less than 100 mVchange in the LED voltage when switch 118 is running at 1 MHz with a 500mA pulse current.

The “on” time of switch 118 is referred to as the duty cycle or “D” andthe “off” time of switch 118 is “1-D.” Because buck-boost converter 100only delivers current to LEDs 108 during 1-D, the average current inLEDs 108 is equal to the average current in inductor 104 during D (asmeasured through sense resistor 120) multiplied by 1-D.

Exemplary Floating Buck Power Converter

FIG. 3 is a simplified block diagram of an exemplary floating buck powerconverter circuit 300 for driving light emitting devices, such as LEDs.Converter circuit 300 has a similar topology as the buck-boost convertercircuit 100; except inductor 104 is coupled between LEDs 108 and switch118 and diode 106 are each coupled in parallel with LEDs 108.

Converter circuit 300 has all the benefits of the buck-boost convertercircuit 100 shown in FIG. 1. However, since the floating buck convertertopology of FIG. 3 only works when the input voltage is higher than theoutput voltage, there will be a portion of time where converter circuit300 is off and the power factor will be poorer compared to convertercircuit 100. When using converter circuit 300, LEDs 108 are always onwhen there is current in inductor 104. The corresponding waveforms forthe floating buck converter topology are illustrated in FIG. 4.

When inductor 104 in the floating buck topology is storing energy, thecurrent also flows through LEDs 108. When switch 118 is off, inductor104 discharges its energy in the form of current through LEDs 108. Thus,LEDs 108 see the DC current plus a ripple current in inductor 104. Theripple current is reasonably small and can be further reduced by placinga small capacitor in parallel with LEDs 108, although it is notnecessary in most applications.

Exemplary Waveform Generator

Converters 100, 300 use waveform generator 110 to generate an averagecurrent in LEDs 108 that follows the full-wave rectified AC inputvoltage waveform. There are several methods that can be implemented bywaveform generator 110 to ensure proper phasing of the AC load to createa high quality PFC. Other methods are also possible.

Method #1

In a first method, the AC input voltage can be measured and used tocreate the “PWM inside a PWM” waveforms. A resistor divider network canmeasure voltage from the rectified AC input voltage. The PWM duty cycleis a duty cycle proportional to the measured voltage. The ratio of thevoltage to duty cycle can be scaled (in real time) to allow dimming ofthe LED brightness.

Method #2

In a second method, zero crossing points of the AC input voltage isdetected and used to synthesize a rectified voltage to be used forgenerating the PWM signal. With knowledge of the zero-to-zero crossingpoints, the period of the rectified sine wave can be determined. Theamplitude of the rectified voltage can be set based on a brightnessrequirement for the application.

Method #3

In a third method, PWM current ramp rate is phase-locked to therectified AC input voltage waveform. The ramp rate to charge inductor104 in the floating buck topology is proportional to the voltage on therectified AC input voltage. The maximum and minimum ramp rates can bedetected. For example, the minimum ramp rate corresponds to the valleys(or zero crossing points) of the input voltage waveform and the maximumramp rate corresponds to the peaks of the input voltage waveform. Thedetermined maximum and minimum ramp rates can be used to phase correctlythe “PWM signal inside the PWM signal.”

Exemplary PFC LED Driver Process

FIG. 5 is a flow diagram of process 500 for driving an output load(e.g., LEDs). Process 500 can be implemented by converters 100 or 300,as described in reference to FIGS. 1 and 3.

Process 500 can begin by generating a first signal for commanding aswitch to allow an inductor coupled to the output load transfer energyprovided by an input voltage source to the output load. The first signalhas a first frequency with a duty cycle in proportion to the inputvoltage amplitude, and it commands the switch to make the average inputcurrent to be proportional to the input voltage amplitude (502). In someimplementations, the first signal can be a PWM signal generated by awaveform generator. The first signal can be controlled to have the sameshape and phase as the input voltage waveform.

Process 500 can continue by generating a second signal for commandingthe switch. The second signal has a second frequency that is higher thanthe first frequency (504). For example, the second frequency can beorders of magnitude higher than the first frequency.

Process 500 can continue by commanding the switch with the second signalbased on a value of the first signal (e.g., when the first signal ishigh) (506), thereby providing the output load with substantiallyconstant current pulses, resulting in a substantially constant, pulsecurrent in the output load. When the output load includes one or moreLEDs, the substantially constant pulse current helps prevent colorshifting of the LEDs.

While this document contains many specific implementation details, theseshould not be construed as limitations on the scope what may be claimed,but rather as descriptions of features that may be specific toparticular embodiments. Certain features that are described in thisspecification in the context of separate embodiments can also beimplemented in combination in a single embodiment. Conversely, variousfeatures that are described in the context of a single embodiment canalso be implemented in multiple embodiments separately or in anysuitable sub combination. Moreover, although features may be describedabove as acting in certain combinations and even initially claimed assuch, one or more features from a claimed combination can, in somecases, be excised from the combination, and the claimed combination maybe directed to a sub combination or variation of a sub combination.

What is claimed is:
 1. A single stage, primary side, Power FactorCorrection (PFC) driver circuit, the circuit comprising: a rectifiercircuit configured to rectify an alternating input voltage; an inductorcoupled in series with the rectifier circuit; a diode coupled in serieswith the inductor and an output load; a switch coupled to the inductor;a switch control circuit coupled to the switch and configured for:generating a first signal for commanding the switch to allow theinductor to transfer energy provided by a rectified input voltage to theoutput load, the first signal having a first frequency with a duty cyclein proportion to an amplitude of the rectified input voltage; generatinga second signal for commanding the switch, the second signal having asecond frequency that is higher than the first frequency; and commandingthe switch with the second signal based on the first signal, thecommanding causing a substantial constant pulse current to be providedto the output load.
 2. The circuit of claim 1, where the rectifiercircuit is a diode bridge.
 3. The circuit of claim 1, where the diode isa Schotky diode.
 4. The circuit of claim 1, further comprising: acapacitor coupled in parallel with the output load.
 5. The circuit ofclaim 1, where the output load includes one or more light emittingdiodes.
 6. The circuit of claim 1, where the switch is a powertransistor.
 7. The circuit of claim 6, where the command terminal is agate of the power transistor.
 8. The circuit of claim 1, where theswitch control circuit further comprises: a logic circuit configured forreceiving a first waveform and a second waveform and providing an outputbased on the first waveform and the second waveform; a latch circuithaving a first input coupled to the output of the logic circuit and anoutput coupled to a command terminal of the switch, the latch circuitconfigured for setting a command voltage on the command terminal basedat least in part on the output of the logic circuit; and a comparatorhaving a first input coupled to the switch, a second input coupled to areference voltage and an output coupled to the latch circuit, where thecomparator is configured for resetting the latch circuit based on acomparison of the first and second comparator inputs.
 9. The circuit ofclaim 8, further comprising: a sense resistor coupled to the switchdevice and the first input of the comparator.
 10. The circuit of claim8, where the latch is a flip-flop.
 11. A single stage, primary side,Power Factor Correction (PFC) driver circuit, the circuit comprising: arectifier circuit configured to rectify an alternating input voltage; adiode coupled in parallel to the rectifier circuit and an output load;an inductor coupled in series with the output load; a switch coupled tothe inductor; a switch control circuit coupled to the switch andconfigured for: generating a first signal for commanding the switch toallow the inductor to transfer energy provided by the rectified inputvoltage to the output load, the first signal having a first frequencywith a duty cycle in proportion to an amplitude of the rectified inputvoltage; generating a second signal for commanding the switch, thesecond signal having a second frequency that is higher than the firstfrequency; and commanding the switch with the second signal based on thefirst signal, the commanding causing a substantial constant pulsecurrent to be provided to the output load.
 12. The circuit of claim 11,where the rectifier circuit is a diode bridge.
 13. The circuit of claim11, where the switch is a power transistor.
 14. The circuit of claim 11,where the diode is a Schotky diode.
 15. The circuit of claim 11, furthercomprising: a capacitor coupled in parallel with the output load. 16.The circuit of claim 11, where the output load includes one or morelight emitting diodes.
 17. The circuit of claim 11, where the switchcontrol circuit further comprises: a logic circuit configured forreceiving a first waveform and a second waveform and providing an outputbased on the first waveform and the second waveform; a latch circuithaving a first input coupled to the output of the logic circuit and anoutput coupled to a command terminal of the switch, the latch circuitconfigured for setting a command voltage on the command terminal basedat least in part on the output of the logic circuit; and a comparatorhaving a first input coupled to the switch, a second input coupled to areference voltage and an output coupled to the latch circuit, where thecomparator is configured for resetting the latch circuit based on acomparison of the first and second comparator inputs.
 18. The circuit ofclaim 17, further comprising: a sense resistor coupled to the switchdevice and the first input of the comparator.
 19. The circuit of claim17, where the latch is a flip-flop.
 20. The circuit of claim 17, wherethe command terminal is a gate of the power transistor.
 21. A method ofdriving a light emitting diode (LED), comprising: generating a firstwaveform for commanding a switch to allow an inductor coupled to the LEDto transfer energy provided by an input voltage to the LED, the firstwaveform having a first frequency with a duty cycle in proportion to anamplitude of the input voltage; generating a second waveform forcommanding the switch, the second waveform having a second frequencythat is higher than the first frequency, where a ratio of the inputvoltage to the duty cycle of the second waveform is scaled to allowdimming of LED brightness; and commanding the switch with the secondwaveform based on the first waveform to provide a substantially constantcurrent pulse to the LED.
 22. A method of driving a light emitting diode(LED), comprising: generating a first waveform for commanding a switchto allow an inductor coupled to the LED to transfer energy provided byan input voltage source to the LED, the first waveform having a firstfrequency with a duty cycle in proportion to an amplitude of the inputvoltage source; generating a second waveform for commanding the switch,the second waveform having a second frequency that is higher than thefirst frequency, where the duty cycle of the second waveform isgenerated by: detecting a zero crossing point of the input voltagesource; synthesizing a voltage for generating the second waveform usingthe detected zero crossing point; and commanding the switch with thesecond waveform based on the first waveform to provide a substantiallyconstant current pulse to the LED.
 23. The method of claim 22, where anamplitude of the synthesized voltage can be set based on a brightnessrequirement for the LED.
 24. A method of driving a light emitting diode(LED), comprising: generating a first waveform for commanding a switchto allow an inductor coupled to the LED to transfer energy provided byan input voltage to the LED, the first waveform having a first frequencywith a duty cycle in proportion to an amplitude of the input voltage;generating a second waveform for commanding the switch, the secondwaveform having a second frequency that is higher than the firstfrequency; and commanding the switch with the second waveform based onthe first waveform to provide a substantially constant current pulse tothe LED, where a ramp rate for current charging the inductor isphase-locked to the input voltage, and maximum and minimum ramp ratescorresponding to peaks and valley, respectively, of the input voltage,are used to phase the first and second waveforms.